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LT1432

APPLICATI

I FOR ATIO

–

AUXILIARY

WINDING

AUXILIARY

OUTPUT

+5V OUTPUT

MAIN

WINDING

POSITIVE

REGULATED

OUTPUT

NEGATIVE

REGULATED

OUTPUT

POSITIVE

REGULATOR

POSITIVE

REGULATOR

LT1432 F20

Figure 20

Figure 20 shows how to connect the auxiliary windings.

Dots indicate winding polarity. Pay attention here -- his-

tory shows that with a 50% chance of connecting up the

auxiliary correctly when you ignore the dots, in actual

practice you will be wrong 90% of the time.

The floating output can have either end grounded, depend-

ing on the need for a positive or negative output. Also

shown are the connections for both positive and negative

outputs using a linear regulator. Note that the two circuits

are identical! The floating auxiliary winding allows the use

of a positive low-dropout regulator for negative outputs.

These positive regulators are more readily available, espe-

cially at lower current levels.

There is a way to “cheat” somewhat on auxiliary power for

positive outputs higher than the 5V main output. The

auxiliary winding return can be connected to the 5V

output. This reduces the winding voltage so that more

current is available, and at the same time it actually adds

a load to the 5V output to bootstrap itself. Figure 21 shows

maximum current out of a 14V auxiliary (used to power a

12V linear regulator) connected in this fashion. The aux-

iliary winding voltage is actually 9V. Note that for lighter 5V

loads, there is an inflection point in the curves at about

11V. That is because theoretically the bootstrapping effect

should allow one to draw

unlimited power from the

auxiliary winding when duty cycle exceeds 50%. The

actual available current above 50% duty cycle is limited by

parasitic losses. At high 5V loads, the inflection disap-

pears for the same reason. The curves asymptotically

approach 1 amp at high input voltage because the criteria

used to generate the curves was a drop in auxiliary output

voltage to 13.5V, and again parasitic resistance limits

output current.

Auxiliary windings deliver current in triangular or quasi-

square waves only during switch off time. Therefore the

amplitude of these pulses will be somewhat higher than

the DC auxiliary load current, especially at low input

voltage. This means that in the “stacked” connection,

ripple voltage on the 5V output will increase with auxiliary

load current.

INPUT VOLTAGE (V)

0.2

0.4

0.6

0.8

1.0

LT1432 F21

14V LOAD INCREASED

UNTIL V = 13.5V

I(+5) = 1A

I(+5) = 400mA

I(+5) = 200mA

I(+5) = 50mA

I(+5) = 200mA

Figure 21

LT1432CS8 Datasheet(PDF) 23 Page - Linear Technology

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